There are cemented carbide (e.g., tungsten carbide-based materials with a cobalt binder) cutting inserts that exhibit a surface zone of non-stratified binder enrichment such as disclosed in U.S. Pat. No. 4,610,931 (and U.S. Reissue Pat. No. 34,180) to Nemeth et al. and U.S. Pat. No. 5,955,186 to Grab.
U.S. Pat. No. 4,548,786 to Yohe discloses a process for making a cemented carbide cutting insert with surface binder enrichment wherein a dewaxed blank that does not contain nitrogen is exposed during the heating process to an atmosphere with a nitrogen partial pressure. PCT Patent Publication No. 98/16665 to Lindskog et al. discloses a cemented carbide cutting insert with surface binder enrichment which uses a nitrogen atmosphere for a part of the process. European Patent No. 0 569 696 to Uchino et al. pertains to a cemented carbide cutting insert that contains zirconium and/or hafnium and has a zone of surface binder enrichment underneath the cutting edge. European Patent No. 0 603 143 to Gustafson et al. discloses a method for producing a coated cemented carbide with a zone of stratified binder enrichment that includes sintering a compacted body containing nitrogen in an inert atmosphere (or a vacuum) followed by a cooling at a specific rate.
Kennametal KC850 grade coated cutting insert (KC850 is a registered trademark of Kennametal Inc. of Latrobe, Pa., USA, for cutting inserts) has a zone of stratified binder enrichment. The Nemeth et al. article entitled “The Microstructural Features and Cutting Performance of the High Edge Strength Kennametal Grade KC850”, Proceedings of Tenth Plansee Seminar, Reutte, Tyrol, Austria, Metalwerke Plansee A.G. (1981), pages 613–627 describes the Kennametal KC850 grade cutting insert. The article by Kobori et al. entitled “Binder Enriched Mayer Formed Near the Surface of Cemented Carbide”, Funtai oyobi Funtai Yakin, Vol. 34, No. 3, pages 129–132 (1987) describes stratified binder enrichment.
Other articles discuss the occurrence of a zone of binder enrichment in cemented carbides. These articles include Schwarzkopf et al., “Kinetics of Compositional Modification of (W,Ti)C—WC—Co Alloy Surfaces”, Materials Science and Engineering, A105/106 (1988) pages 225–231, Gustafson et al., “Binder-Phase Enrichment by Dissolution of Cubic Carbides”, Int. J. of Refractory Metals & Hard Materials, 12 (1993–1994), pages 129–136, Suzuki et al., “The B-Free Layer Formed Near the Surface of Sintered WC—B—Co Alloy Containing Nitrogen”, Nippon Kinzoku Gakkaishi, Vol. 45, No. 1 (1981), pages 95–99, and Suzuki et al., “The B-Free Layer Formed Near the Surface of Vacuum-Sintered WC—B—Co Alloys Containing Nitrogen”, Transactions of the Japan Institute of Metals, Vol. 22, No. 11 (1981), pages 758–764.
While some of the above articles, patents and products disclose or comprise cutting inserts that exhibit adequate performance, there remains a need to develop processes that produce products (and the products themselves) that have better properties. In this regard, it would be desirable to provide a process (and the resultant product) that sinters the blank in an atmosphere most always having at least a partial pressure so as to be able to control the depth of the zone of binder enrichment. Such a process would provide for an optimum balance between the edge strength and the deformation resistance of the substrate. Such a process would also provide for excellent consistency in the depth of the zone of binder enrichment for the parts throughout a heat.
It would also be desirable to provide a process, as well as the resultant product, wherein there is no carbon precipitation in the zone of binder enrichment, especially in a substrate that has a core porosity of greater than C00 according to ASTM Designation B276-91 (Reapproved 1996). The absence of such carbon precipitation would enhance the adhesion of the coating to the substrate.
It would be advantageous to provide an as-sintered cemented carbide that exhibits a surface zone of non-stratified binder enrichment (or essentially non-stratified binder enrichment which means that most of the binder enrichment is of the non-stratified type with a slight (or small) amount of stratified binder enrichment) wherein there is enhanced solid-solution hardening. In this regard, a cemented (cobalt) tungsten carbide substrate that has nitrogen atoms present at the interstices of the cobalt atoms facilitates solid-solution hardening. The enhancement of solid-solution hardening is especially true for a substrate that with a bulk region that exhibits a porosity of greater than C00 according to ASTM Designation B276-91 (Reapproved 1996). In such a case, the atomic radius of nitrogen (about 0.75 Angstroms) is smaller than the atomic radius of carbon (about 0.91 Angstroms).
It would be advantageous for applying a coating, and especially a coating that contains nitrogen (e.g., titanium nitride or titanium carbonitride), directly on the surface of a substrate that contains nitrogen. In the case of the application of a coating of titanium nitride on the surface of a substrate that has bulk region with a porosity of not greater than C00 according to ASTM Designation B276-91 (Reapproved 1996), the presence of nitrogen would promote nucleation of titanium nitride. In the case of the application of titanium carbonitride to the surface of a substrate with a bulk region exhibiting a porosity of greater than C00 according to ASTM Designation B276-91 (Reapproved 1996), the presence of carbon and nitrogen would help promote the nucleation of titanium carbonitride.
It is believed that with the presence of additional nitrogen in the cobalt binder for a cemented (cobalt) tungsten carbide substrate that has a surface zone of cobalt enrichment, there is an increase in the chemical affinity between the substrate and a nitrogen-containing coating, such as, for example, titanium nitride or titanium carbonitride. It is believed that such an increase in the chemical affinity should lead to an increase in the adhesion of the coating to the substrate.
It is believed that an increase in the availability of nitrogen in the cobalt near the surface of the substrate should reduce the potential for the formation of a brittle eta phase at the interface between the coating and the substrate. The reduction in the potential to form eta phase permits the use of substrates that have lower carbon contents.
It is believed that a higher nitrogen content in the substrate should also result in a decrease in the grains size of the tungsten carbide. An increase in the N/(C+N) content should lead to a decrease in the grain size of the tungsten carbide. The tungsten carbide phase content in the microstructure should increase to a maximum as the N/(C+N) ratio increases.
It can thus be seen that there is a belief that it would be advantageous to provide an as-sintered cemented (cobalt) tungsten carbide substrate that has a higher nitrogen content. The higher nitrogen content should increase adhesion strength between the coating (especially a coating such as titanium nitride and titanium carbonitride) and the substrate. The higher nitrogen content in the cobalt binder near the surface of the substrate should reduce the potential for the formation of brittle eta phase at the coating-substrate interface. The higher nitrogen content should decrease the grain size of the tungsten carbide.
Typically, it has been necessary to use different compositions of the starting powder to produce either an as-sintered substrate that exhibits a surface zone of binder enrichment or an as-sintered substrate in which there is an absence of a surface zone of binder enrichment. As can be appreciated, there is an increase in the cost associated with storing (and/or making) two or more different compositions of starting powder as compared with the cost of storing (and/or making) only one composition of starting powder. From a production viewpoint, it would advantageous to provide a process that would utilize a single starting powder composition to selectively produce either an as-sintered substrate of a commercial quality with a surface zone of binder enrichment or an as-sintered substrate of a commercial quality that does not have a surface zone of binder enrichment.